This invention relates to spot welding apparatus, particularly to so-called pogo welding with a single electrode welding gun.
It is known in the art relating to spot welding to connect the central portions of large sheet assemblies by so-called pogo welding. In this method, a welding gun having a single electrode is moved sequentially to a series of welding locations at each of which spot welding is performed by transmitting current from the welding gun electrode through the sheets to be joined to backup electrodes located below the specified welding locations.
FIG. 1 shows the concept of a prior pogo welding apparatus 10, which includes a programmable robot 12 having a linearably movable base 13 supporting a movable jointed arm 14 carrying a welding gun 16 with a single pogo electrode 18. The electrode is connected electrically with a transformer 20, also connected with an electrically conductive stand 22. The stand supports a plurality of copper backup electrodes 24, each of which engages one of selected welding locations 25 on the lower side 26 of a temporary or loose assembly of two metal sheets, supported on the stand to be spot welded into a permanent assembly 28.
In operation, the pogo electrode 18 is moved sequentially by the robot 12 to each of the selected locations 25 on the opposite upper side 30 of the assembly 28. There, the electrode engages the upper side 30 opposite from each of the backup electrodes 24 in sequence. At each location 25, the welding gun 16 is energized to conduct welding current through the assembly 28 to the opposite backup electrode 24, spot welding the metal sheets together.
In this prior arrangement, a part of the welding current travels through the metal sheets to the neighboring backup electrodes 24 while the remainder passes through the assembly between the opposing electrodes to perform the spot welding process. Significant current shunting may lead to discrepant welds. The prior process requires a different table or tool tray specifically designed to engage the selected locations for each differing component design that is to be spot welded. Thus, a new or modified component requires provision of a modified welding fixture with the backup electrodes positioned in the changed locations.
The present invention replaces the conductive stand or tray with a suitable support, where needed, and a programmable positioner carrying a single backup electrode. The support holds the metal sheets in temporary assembly during the spot welding procedure. The positioner carries the single backup electrode on a movable head, which is universally movable within a desired envelope. The positioner is programmed to move the backup electrode sequentially to the various selected locations to be spot welded so that the backup electrode is positioned against one side of the sheet assembly at selected locations when the robot carried pogo electrode is moved against the opposite side of the assembly at the respective selected locations. Various types of positioners may be utilized which may be suitable for the particular components to be welded.
Unlike, the prior table or tray with multiple prepositioned backup electrodes arranged for use with a single component design, the positioner of the invention may be used with differing component designs as may be desired by merely reprogramming the controller for the positioner instead of requiring a new or modified electrode tray or table. The invention further eliminates shunting of welding currents away from the active welding location, since only two opposing electrodes are provided for conducting current through each of the selected welding locations. Thus, weld quality becomes more consistent and the fixture costs resulting from design changes are substantially reduced.
A modification of the invention includes a multi head back-up electrode usable in the inventive apparatus and method to facilitate contact with interior back-up locations that would otherwise be more difficult or less efficient to reach.
These and other features and advantages of the invention will be more fully understood from the following description of certain specific embodiments of the invention taken together with the accompanying drawings.